1. Field of the Invention
This invention relates to high surface area iron-cobalt Fischer-Tropsch slurry catalysts, their preparation and use in Fischer-Tropsch slurry processes for selectively producing high amounts of C.sub.2 to C.sub.20 alpha-olefin materials.
2. Brief Description of the Prior Art
Methods for preparing low molecular weight olefins by Fischer-Tropsch processes using coprecipitated iron-based catalysts including cobalt as cocatalyst, are well-known in the art, as described, for example, in U.S. Pat. Nos. 2,850,515; 2,686,195; 2,662,090; and 2,735,862; AICHE 1981 Summer Nat'l Meeting Preprint No. 408, "The Synthesis of Light Hydrocarbons from CO and H.sub.2 Mixtures over Selected Metal Catalysts" ACS 173rd Symposium, Fuel Division, New Orleans, March 1977; J. Catalysis 1981, No. 72(1), pp. 37-50; Adv. Chem. Sem. 1981, 194, 573-88; Physics Reports (Section C of Physics Letters) 12 No. 5 (1974) pp. 335-374; U.K. patent application No. 2050859A; J. Catalysis 72, 95-110 (1981); Gmelins Handbuch der Anorganische Chemie 8, Auflage (1959), pp 88-96; and Chem. Ing. Tech. 49 (1977) No. 6, pp. 463-468.
It is further known that high levels of cobalt in coprecipitated iron-cobalt alloy catalysts produce enhanced selectivity to olefinic products under certain process conditions, as described in Stud, Surf. Catal. 7, Pt/A, pp. 432 (1981).
Although the above-described prior art describes catalysts and processes displaying good fixed bed olefin synthesis activity, what is particularly desired are slurry catalysts which can preferably be completely pretreated in situ in the slurry liquid to yield the reduced, carbided active catalysts in the process displaying the combination of good C.sub.2 -C.sub.20 olefin synthesis activity, low selectivity to methane, coupled with long-term activity maintenance which is essential for a successful commercial process. Particularly desired is where the catalyst precursor is the metal oxide spinel of the final catalyst composition.
It has been found that low surface area iron-cobalt spinels having BET surface areas below 5 m.sup.2 /g are not readily pretreated in situ in a Fischer-Tropsch slurry liquid under mild conditions to readily yield active catalysts for producing C.sub.2 -C.sub.20 olefins.
The preparation of high surface metal oxides is described in the French article, "C. R. Acad. Sc. Paris", p268 (May 28, 1969) by P. Courte and B. Delmon. The article describes a process for producing high surface area metal oxides by evaporating to dryness aqueous solutions of the corresponding glycolic acid, lactic acid, malic or tartaric acid metal salts. One oxide that was prepared by their described method was CoFe.sub.2 O.sub.4.
However, the above references do not describe or suggest the use of single phase Fe:Co spinels having iron-cobalt atomic ratios of 4:1 or above or suggest their applicability in conducting or carrying out slurry-type Fischer-Tropsch processes.